If you want AVs as horde, que with atleast 1 ally also. There is no need to join the bg with the ally character, just let your spot be replaced with someone that ques for random bgs.
If everyone who wants avs to happen do this it will pop. If you dont do this there is no point in que for only av.
The time it takes to level up an ally to 20 wont be any longer than the time you spend queing and get no pops.